It is known that, in the course of the primary processing thereof, the synthetic textile filaments (endless yarns, strands, tows and the like) obtainable from certain polymeric starting materials, such as polyamides, polyesters and polyacryls, are first spun from the melt or from solution and are then stretched in the presence of heat or heat and moisture to improve the textile qualities thereof. Stretching is generally followed by more or less intensive thermal or hydrothermal fixing of the filaments to bring about the reduction in the residual shrinkage thereof which is required for further applications.
For certain applications, for example for tyre cord, for marine ropes, for filter cloths, for parachute silk, for safety belts and for fiber reinforcements in plastics articles to increase the breaking strength thereof, high-tensile synthetic filaments and fibers compete with metal filaments and natural fibers.
Materials suitable for the production of high-strength filaments are polymeric materials, such as polyesters (polyethylene terephthalate) and polyamides (polyamide-6, polyamide-6,6), because tensile strengths of from 7 to 9.5 cN/dtex (from 95 to 130 daN/mm.sup.2) may be obtained in the case of polyester filaments and of from 6 to 9 cN/dtex (from 70 to 100 daN/mm.sup.2) in the case of polyamide-6 filaments by maximum stretching. Providing they are stretched to high levels at suitable temperatures, the filaments also have a high modulus of elasticity. Technical polyester filaments, for example, have modulus values of from 70 to 120 cN/dtex, while technical polyamide-6 filaments have modulus values of from 60 to 90 cN/dtex.
The above-mentioned applications requiring high tensile strength and modulus values are largely not applicable to polyacrylonitrile filaments (PAN-filaments) because it is only possible to obtain tensile strength values of from 3.5 to 4.0 cN/dtex and modulus values of from 30 to 50 cN/dtex.
However, it is known from Japanese Patent Application No. 53-139824 of 1978 that fibers having a maximum tensile strength of 6.5 cN/dtex may be obtained from wet-spun PAN containing at least 90% of acrylonitrile (spun from a 2000-bore spinneret according to Example 1) by stretching in hot water followed by further stretching in steam under an excess pressure of from 0.5 to 5 bars (the total stretching ratio .gamma. amounting to 18) and then by drying at from 105.degree. to 125.degree. C.
In the case of wet-spinning, the filaments are taken up at low speeds (for example 5 m/min.). The spun material is at most slightly pre-oriented and may at most be very highly stretched. Due to the different filaments structure thereof, as reflected inter alia in the filament cross-section ("bone-shaped" as opposed to the circular cross-section of wet-spun filaments) and in the distinctly higher orientation imparted during spinning, dry-spun PAN which is taken up at much higher speeds, generally of the order of from 200 to 400 m/min., cannot be as highly stretched as wet-spun material according to the Japanese Patent Application and, in addition, it has a distinctly lower tensile strength.
In addition, it is known from DE-OS No. 2,851,273 that wet-spun PAN containing at least 70%, by weight, of acrylonitrile may be processed by extruding the spinning solution through a 10-bore spinneret into an aqueous precipitation bath to form filaments, drawing the filaments from the precipitation bath in a first stretching stage, washing the drawn filaments to remove the precipitation medium and, drawing the washed filaments in water at from about 70.degree. to 100.degree. C. (2nd stretching stage), the total stretching ratio 65 of the 1st and 2nd spinning stages amounting to from 3 to 14. This known process is characterised in that the filaments obtained are subequently passed through an excess-pressure steam stretching zone having a vapor pressure sufficient for a temperature of from about 110.degree. to 140.degree. C. in which the filaments are drawn in a ratio .gamma. of at least about 20 to increase the total stretching ratio. The finished fibers have a high linear strength of at least 10 g/den (9 cN/dtex) and a high initial modulus of at least 120 g/den (110 cN/dtex).
Attempts have also been made to subject endless yarn of dry-spun PAN to an optimal after-treatment to obtain high tensile-strength values. Thus, according to DE-OS No. 1,939,388, individual filament tensile strengths of at most from 7.5 to 8.0 g/den from 6.6 to 7.1 cN/dtex are obtained from spun material containing from 20 to 40 individual filaments by stretching in steam (0.9 m tube; 100.degree. C.; .gamma..sub.1 =1.6), washing and drying, followed by further stretching in a ratio .gamma..sub.2 of 7 over a hot "shoe".
According to DE-OS No. 2,658,916, high-strength endless yarns having a final overall titre of from 20 to 145 tex may be obtained from dry-spun PAN. In this known process, the filament material (pre-stretched to a certain extent at the spinning stage) is stretched on a heating godet/heating bow combination at temperatures of the order of 145.degree. C., after which the stretched filaments are twisted and treated with steam under pressure at 125.degree. C. after winding into a package form. The finished yarns have tensile strengths of at least 4.7 cN/dtex and at most 5.35 cN/dtex.